Field of the Invention
The present invention concerns a novel device intended to improve conditions for carrying out extraction and/or separation and/or cracking processes by means of supercritical fluids. It should first of all be recalled that when a pure fluid or a mixture is subject to conditions beyond the critical point C, a change of state exists between the gaseous phase (or vapour) and the liquid phase encountered during both liquefaction and evaporation/boiling. Once this critical point C has been exceeded, this state change disappears in order to allow to exist a single phase supercritical fluid but characterized among others by wide variations of volumic mass .rho.. If reference is made to the diagrams of FIGS. 1 and 2 illustrating the various states of the state changes as a function (FIG. 1) of the temperature T and the pressure P and (FIG. 2) of the density .rho. and of the pressure p, it can be seen (FIG. 1) that if a pressure p.sub.1 prevails comprised between the critical pressure P.sub.c and the pressure P.sub.t at the triple point T, when the temperature is increased, there is a change from the liquid phase 1 to the gaseous phase g. If a pressure P.sub.2 prevails higher than the critical pressure P.sub.c, there is no state change by increasing the temperature but the field of the supercritical fluid subsists (here will be mentioned simply the solid phase which does not concern the present invention).
FIG. 2 illustrates that it is possible to cross successively at pressures P.sub.1 (sub-critical) and P.sub.2 (supercritical) of the isotherm curves: T.sub.c corresponding to the critical temperature, T.sub.1 at a sub-critical temperature and T.sub.2 at a supercritical temperature. At a given pressure, it can be seen that for sub-critical P.sub.1, a sudden and great change of density .rho. is encountered when there is a change from the gas g to the liquid l (or vice versa) with a temperature plateau and discontinuity in the variation of volumic mass of each phase (Pg.sub.1, Pl.sub.1) to the state change whereas for the supercritical pressure P.sub.2 no further change is encountered and the corresponding sudden variation of density, but great and continuous variations of density .rho. as a function of temperature.
The extraction-separation-cracking processes by supercritical fluids are based on the fact that these fluids present outstanding properties with respect to liquids, especially a lower viscosity and a greater diffusivity: they present a further interesting property, namely the characteristics of their solvent power; therefore, when there is a change from the subcritical gaseous state to the supercritical fluid state, not only very high variations of density are encountered as explained herein-above, but there is also a considerable increase in the solubility of the third body; furthermore, contrary to the liquid-liquid or liquid-solid extraction, the final solvent-extract separation does not constitute a difficult and expensive step, for example, such as re-extraction with a second solvent or distillation, but can be easily carried out by simple isotherm expansion or close to isothermicity or by heating at constant pressure or by combination of these two previous methods, the sudden variation of the solvent power and the demixtion of the extract being obtained by a sharp decrease in the volumic mass.
Certain of these properties of the supercritical fluids have been utilized in the prior art with a view to use in separation, extraction and cracking and in particular chromatographic processes (cf. French patent application filed under No. 82 09649 dated June 3, 1982).
To be more specific, three embodiments featured in FIGS. 3 to 5, will be given by way of example, the figures having the index "a" representing the flow diagram, the figures having the index "b" representing on the enthalpic diagram the corresponding cycles. On FIGS. 3a to 5a, the points or equipment designated by the same letters on the enthalpic diagrams 3b to 5b have been designated by captital letters. On these same diagrams, the separation step is represented in a dotted line on the cycle. On FIGS. 3 and 4, this solvent-extract separation is carried out by pressure drop and heat contribution in order to prevent cooling due to expansion, but the recycling of the solvent differs. On FIG. 3, a pump P is used to raise the pressure of the condensed sub-critical liquid and to bring it to the supercritical fluid state. On FIG. 4, a compressor K is used to bring the sub-critical gas to the supercritical state then an exchanger Q' to cool the fluid to the desired temperature.
On FIG. 5, the solvent-extract separation is carried out by temperature rise, the recycling of the solvent being carried out by means of a compressor K working at a low rate of compression in established state.
To be more explicit, the three following cases will now be considered separately: